Generally, nitrogen oxides are generated from a stationary source such as an industrial boiler, a gas turbine, a steam power plant, a waste incinerator, a marine engine, and a petrochemical plant. A technology of removing nitrogen oxides may be classified into the following three methods. Firstly, a fuel denitrification method includes treating a fossil fuel to remove nitrogen compounds contained therein. A second method includes improving a combustion condition. At this time, the improvement of the combustion condition may be accomplished through an excess air feeding and a multi-stage combustion process in consideration of the type of a fuel. Finally, a post-treating method includes treating an exhausted gas to remove nitrogen oxides.
In the fuel denitrification method, even though the fossil fuel is treated at relatively high temperatures under hydrogen for a long time in order to remove nitrogen oxides contained in a coal, only about 16% of total nitrogen oxides content is removed. Additionally, in the case of the second method to improve the combustion condition, it is impossible to remove nitrogen oxides in efficiency of 30-40% or more because an exhaustion condition of nitrogen oxides is inversely related to a thermal efficiency.
Among the three methods, the post-treatment is sufficiently competitive in terms of removing efficiency of nitrogen oxides, thus being commercialized.
The post-treatment is roughly classified into wet and dry treating methods. In this regard, the wet treating method has advantages in that nitrogen oxides and sulfur oxides are simultaneously removed, and thus is applied to a process in which a small amount of nitrogen oxides is emitted. However, it is required to oxidize NO into NO2 because a solubility of NO in water is poor, thus not securing economic efficiency. In addition, undesirably, NO3 and N2O4 generated as a side product during oxidizing NO into NO2 should be re-treated.
Accordingly, the dry treating method is being watched with keen interest. The dry treating method is classified into a selective non-catalytic reduction (SNCR) process in which nitrogen oxides are selectively reduced into nitrogen and moisture by spraying ammonia into nitrogen oxides at a relatively high temperature ranging from about 850 to 1050° C. without using a catalyst, and a selective catalytic reduction (SCR) process in which nitrogen oxides are reduced into nitrogen and moisture at a relatively low temperature of about 150 to 450° C. using a catalyst. The SNCR process has an advantage in that 50% or more of nitrogen oxides are removed at relatively low costs, but has disadvantages in that unreacted ammonia forms ammonium salts, thus plugging or corroding a device positioned after a reactor. Further, a narrow operation temperature range is still problematic. Therefore, the selective catalytic reduction is being considered as a useful approach for removing nitrogen oxides generated from a stationary source in views of economic and technological efficiency.
In the SCR process, nitrogen oxides such as nitrogen monoxide (NO) and nitrogen dioxide (NO2) are reduced into nitrogen and moisture using ammonia as a reducing agent in the presence of the catalyst, as shown in the following Reaction equations 1 to 4. At this time, an exhausted gas contains oxygen as well as nitrogen oxides, thus, practically, the reduction of nitrogen oxides is accomplished according to the Reaction equations 3 and 4.6NO+4NH3→5N2+6H2O  Reaction equation 16NO2+8NH3→7N2+12H2O  Reaction equation 24NO+4NH3+O2→4N2+6H2  Reaction equation 32NO2+4NH3+O2→3N2+6H2O  Reaction equation 4
However, undesirably, ammonia used as the reducing agent reacts with oxygen, thus producing nitrogen and nitrogen oxides, as shown in the following Reaction equations 5 to 8.4NH3+3O2→2N2+6H2O  Reaction equation 54NH3+4O2→2N2O+6H2O  Reaction equation 64NH3+5O2→4NO+6H2O  Reaction equation 74NH3+7O2→4NO2+6H2O  Reaction equation 8
Usually, the oxidation of ammonia is accelerated with an increase of a temperature, and competes with the reduction of nitrogen oxides. Hence, a conversion of nitrogen oxides depends on the temperature. In the case of the exhausted gas without moisture, the oxidation of ammonia according to the Reaction equation 6 does rarely occur, but nitrogen oxides are generated according to the Reaction equations 7 and 8. At this time, a reaction rate of ammonia with oxygen is increased with an increase in the temperature.
Meanwhile, in case that the exhausted gas contains moisture and sulfur oxides, the moisture and sulfur oxides form salts, thus reducing the activity of the catalyst. The catalyst is poisoned by the moisture and/or the sulfur oxides as set forth in the following Reaction equations 9 to 12.2NH3+H2O+2NO2→NH4NO3+NH4NO2  Reaction equation 92SO2+O2→2SO3  Reaction equation 10NH3+SO3+H2O→NH4HSO4  Reaction equation 11SO3+H2O→H2SO4  Reaction equation 12
In the Reaction equation 9, nitrogen dioxide reacts with ammonia to produce ammonium nitrate. The as-synthesized ammonium nitrate is known to be decomposed at 150° C. or higher. Thus, the catalyst is not poisoned by ammonium nitrate at 150° C. or higher. Practically, ammonia is fed into the exhausted gas at 150° C. or higher, and the catalyst is poisoned by sulfates formed according to the Reaction equation 11, which remain on the catalyst without being decomposed. At this time, such sulfates are produced from sulfur trioxide generated according to the Reaction equation 10. Furthermore, sulfuric acid is produced according to the Reaction equation 12, causing corrosion of a catalyst bed and other devices in a subsequent stage to be corroded.
The production of sulfur trioxide according to the Reaction equation 10 is increased at relatively high temperatures. Thus, there remains a need to develop a catalyst capable of selectively reducing nitrogen oxides at a relatively low temperature window in order to suppress the production of sulfates and sulfuric acid according to the Reaction equations 11 and 12.
Various catalysts from precious metal catalysts to basic metal catalysts have been proposed in the SCR technology. Furthermore, it is reported that supports for the metal catalyst play an important role in the SCR. In this regard, most of the recently developed SCR catalysts include vanadium as an active material, and for example, the desirable SCR performance is obtained by use of a catalyst in which vanadium pentoxide (V2O5) is supported on titania (TiO2), alumina (Al2O3) or silica (SiO2). At this time, the most important one of criteria of the support is the resistance to sulfur. In fact, titania is mainly used as support in commercialized vanadium-contained catalysts. In addition, a catalyst including tungsten or molybdenum is also being developed to suppress sulfur trioxide produced according to the Reaction equation 10.
In order to better understand the background of the invention, a description will be given of conventional technologies for the catalyst containing vanadium as active material.
U.S. Pat. No. 4,152,296 discloses a method of producing a denitrification catalyst comprising impregnating vanadium sulfate (VSO4), vanadyl sulfate (VOSO4), or a mixture thereof onto TiO2 carrier in such a way that at least 0.1%, preferably 0.35 to 1.35% of vanadium element is contained in the catalyst based on a weight of the carrier, and then reacting a mixed gas consisting of ammonia and an inert gas with the impregnated carrier at 300-520° C. The resultant denitrification catalyst has a pore volume of 0.3 to 0.45 cc/g and a specific surface area of 20 to 50 m2/g. Since the mixed gas of ammonia and inert gas is used at the time of calicinating the catalyst in this patent document, but the calcination of a catalyst is conducted in an oxygen atmosphere in the present invention, this patent document is different from the present invention in calcination conditions.
U.S. Pat. No. 4,182,745 discloses a denitrification catalyst having activity at 250 to 450° C., which is produced by impregnating a salt of a transition metal such as Cu, Ti, V, Cr, Mn, Fe, Co, and Ni with a heteropoly acid such as silicotungstic acid, silicomolybdic acid, phosphotungstic acid, and phosphomolybdic acid on a heat-resistant porous material such as alumina, silica, and silica-alumina acting as a carrier, and drying and calcinating the resulting mixture. In this regard, the carrier preferably has a specific surface area of 50 m2/g or more and a pore volume of 0.2 to 1.5 cc/g.
Further, U.S. Pat. No. 4,929,586 discloses a catalyst for removing NOx, in which an active material such as V2O5, MoO3, WO3, Fe2O3, CuSO4, VOSO4, SnO2, Mn2O3, and Mn3O4 is supported on a titania carrier (TiO2) with an anatase crystalline structure. At this time, a conversion of NOx is about 90% at 350° C. The titania carrier has a micropore porosity of 0.05 to 0.5 cc/cc, a macropore porosity of 0.05 to 0.5 cc/cc, and a total porosity of 0.8 cc/cc or lower. In this regard, the micropore porosity means a porosity of pores with a pore size of 600 Å or less, and the macropore porosity means a porosity of pores with a pore size of 600 Å or more.
Furthermore, U.S. Pat. No. 5,045,516 discloses a method of producing a catalyst for removing nitrogen oxides, in which molybdenum trioxide and 10% or less vanadium pentoxide are supported on TiO2. In this regard, TiO2 includes 500 ppm or less calcium, 100 ppm or less iron, and 60% or more anatase crystal, and has a mean particle size of 10 to 100 nm, a mean pore radius of 10 to 30 nm, and a BET surface area of 10 to 80 m2/g to prevent the catalyst from being poisoned by arsenic compounds contained in an exhausted gas.
U.S. Pat. No. 5,753,582 discloses a catalyst comprising a carrier, such as alumina, aluminate, titanium dioxide, or zirconium dioxide, and active metals, such as vanadium oxides, molybdenum oxides, or tungsten oxides, which can be applied at a temperature of 300° C. or higher, and preferably at a high temperature of 350˜450° C.
Moreover, U.S. Pat. No. 6,054,408 discloses a catalyst for removing nitrogen oxides, in which 0.01 to 5 wt % molybdenum trioxide and 0.01 to 5 wt % vanadium pentoxide are supported on an anatase-typed titania (TiO2) carrier. The anatase-typed titania carrier includes 5% or less rutile-typed crystalline structure, 500 ppm or less sodium, 500 ppm or less potassium, 500 ppm or less iron, and 0.5% or less phosphorus.
U.S. Pat. No. 4,952,548 discloses a catalyst for removing nitrogen oxides, the atomic ratio of Ti:Mo and/or W:V being 80-96.5:3-15:0.5-5. Particularly, a size of a TiO2 crystal is limited to prevent a TiO2 surface of the catalyst from being poisoned by heavy metals, and thus has a range of 185 to 300 Å in the direction of plane (101) according to a Sherrer equation.
Furthermore, U.S. Pat. No. 4,916,107 discloses a catalyst for removing nitrogen oxides, which includes titanium oxides, tungsten oxides, and oxides of at least one metal selected from the group consisting of vanadium, iron, niobium, and molybdenum. In detail, an anatase-typed titania (TiO2) is used as a carrier, the catalyst has a specific surface area of 50±15 m2/g, a first mean particle size of 30 nm, a dry loss of 1.5 wt %, and an ignition loss of 2 wt %. In addition, the catalyst comprises 99.5% TiO2, 0.3 wt % Al2O3, 0.2 wt % SiO2, 0.01 wt % Fe2O3, and 0.3 wt % HCl.
The above patents specify physical properties of titania used as the carrier, but do not mention how the properties and states of an active metal oxides supported on the carrier and the reaction participation of lattice oxygen due to the properties and states of the active metal oxides affect the SCR performance, and most of the above patents mention catalysts which can be applied at a high temperature of 300° C. or higher, as in U.S. Pat. No. 5,753,582. Additionally, most of the above patents disclose that vanadium used as the active metal is vanadium pentoxide.
As described above, commercialized denitrification catalysts include tungsten or molybdenum to improve activity and poison resistance to sulfur dioxide of a conventional V2O5/TiO2-based catalyst. Most of denitrification catalysts have optimum activity at a relatively high temperature of 300˜400° C. Therefore, an equipment for removing nitrogen oxides in a thermoelectric power plant or a general boiler is installed in a region where the temperature of flue gas is 300˜400° C. In a thermoelectric power plant, the equipment for removing nitrogen oxides may be provided at the rear end of an economizer in order to secure the temperature of flue gas. In this case, flue gas can be maintained at high temperature, but dust in the flue gas is directly transferred to the denitrification catalyst, thus causing a catalyst to be worn or be plugged. For this reason, when a honeycomb catalyst is mounted, cell size is increased, so that contact area is decreased, thereby increasing the amount of catalyst. Such a system is referred to as a high-dust system. In order to decrease the influence of dust, an electrostatic dust collector or a bag filter may be provided at the front end of a denitrification equipment, but most dust collection systems are operated in normal only when the temperature of flue gas is low. Occasionally, a high-temperature electrostatic dust collector is used, but cost is very high. Such a system is referred to as a low-dust system. This low-dust system is influenced by dust, but is disadvantageous in that the temperature of flue gas is high, and it takes much cost to maintain the temperature of flue gas at high temperature. Moreover, when sulfur oxides are present in flue gas, ammonium sulfate can be formed due to the low flue gas temperature. The ammonium sulfate poisons a catalyst and corrodes the denitrification equipment, thus shortening the life span of the total system. In order to solve the disadvantages, a desulfurization equipment may be provided at the front end of the denitrification equipment. Such a system is referred to as a tail-end system. This tail-end system can remove sulfur oxides and dust from the flue gas flowing into the denitrification equipment. However, in the desulfurization equipment, a wet type desulfurization method or a semi-dry type desulfurization method is used, and the temperature of flue gas is decreased to 100° C. or lower. It is almost impossible to conduct denitrification at these low temperatures. Therefore, the temperature of flue gas must be raised to a temperature at which the denitrification reaction can be conducted using a flue gas reheating apparatus, such as a duct-burner, etc. In practical, it is required to consume a great amount of power corresponding to about five to ten % of a total capacity of a power plant to raise the temperature of the flue gas by approximately 100° C. Even in the low-dust system, flue gas is reheated after the dust collector, and, at this time, auxiliary fuel must be supplied. As described above, a conventional flue gas denitrifying process is a process of removing nitrogen oxides from flue gas using high energy depending on the catalytic activity at high temperature. Therefore, when the conventional equipments are utilized in the denitrifying process, a V2O5/TiO2-based catalyst must be installed in the space of a temperature of about 300˜400° C. at which the V2O5/TiO2-based catalyst is activated. In this case, there are systematic problems and spatial problems depending on installation positions. Furthermore, the denitrifying process at relatively high temperatures is disadvantageous in that it increases the thermal fatigue of a catalytic bed, thus reducing a life span of the catalyst, and promotes the oxidation of sulfur dioxide, thus producing a catalyst poison such as ammonium sulfate.
Hence, unlike the conventional V2O5/TiO2-based catalyst, a catalyst capable of maintaining high catalytic activity at a low temperature of 300° C. or lower is required to reduce auxiliary fuel cost in the low-dust system or tail-end system and to establish an equipment for removing nitrogen oxides, which does not need a flue gas reheating apparatus even if the catalyst is installed at a temperature lower than 300-400° C. Thus, the catalyst is advantageous in that economic efficiency can be improved, denitrification systems can be variously designed and installed, and the problem of a life span of the catalyst due to thermal fatigue can be overcome by suppressing the production of the catalyst poison caused by sulfur oxides and maintaining high catalytic activity even at low temperatures.
Regardless of this requirement, the conventional commercial V2O5/TiO2-based catalyst is activated at a relatively high temperature of 300° C. or higher, but its activity is drastically reduced at a relatively low temperature of 300° C. or lower, so that desired nitrogen oxides removal efficiency is accomplished by installing a flue gas reheating apparatus or increasing the amount of a catalyst. Because an activation energy necessary for a selective catalytic reduction reaction is insufficient at a low temperature range of 300° C. or lower, it is difficult to cause the oxidation/reduction of the catalyst. Accordingly, a conversion of nitrogen oxides is low at 220° C. or lower, and an exhaustion concentration of unreacted ammonia is high. Nitrogen oxides and unreacted ammonia in the flue gas are poisonous to human body, and unreacted ammonia reacts with sulfur compounds and moisture in the flue gas to form ammonium salts, thus deactivating the catalyst. Therefore, it is necessary to develop a catalyst with excellent oxidizing and reducing ability at even relatively low temperature window.